This invention relates to a method for rapidly fabricating complex three-dimensional ceramic, metal and polymer parts, and in particular to a method of making molded parts using complex fugitive molds built using a layered manufacturing process.
The following discussion will treat ceramic part production in detail; many of the problems facing the manufacture of ceramic parts are also encountered in the manufacture of polymer and metal parts.
A number of techniques and processes have been used for making complex three-dimensional ceramic parts. Prior art methods of making relatively simple ceramic parts include powder pressing, slip casting, tape casting, and extrusion. Slip casting and tape casting produce relatively fragile xe2x80x9cgreenxe2x80x9d (not yet sintered) parts. For information on slip-casting, see U.S. Pat. Nos. 4,556,528 and 5,252,273. Extrusion requires high binder content in the green material which leads to lengthy and difficult burn out cycles. Complex parts can be made by machining of green ceramic billets or by injection molding. The abrasivity of ceramic green materials makes machining expensive due to rapid tool and machine wear. Injection molding requires expensive, long-leadtime tooling and produces green parts with high binder content.
Rapid prototyping methods allow the fabrication of complex near-net shapes without the need for tooling. For information on rapid prototyping processes for making green ceramic parts, see xe2x80x9cIndirect Fabrication of Metals and Ceramics,xe2x80x9d Chapter 5 in Solid Freeform Fabrication: A New Direction in Manufacturing, Beaman et al., Dordrecht: Kluwer Academic Publishers, 1997. By producing near-net-shape parts these processes can reduce or eliminate the need for green machining. However, they typically produce parts that exhibit a stairstep surface as a result of being built incrementally in layers. While the outside surfaces of green parts made by such processes could be manually smoothed, there is no way to smooth inaccessible interior surfaces. Since the parts are built in layers there is also a potential for defects at the layer boundaries.
Gelcasting is a relatively new ceramic forming process. For general information on gelcasting see the article by Krause xe2x80x9cORNL""s Gelcasting: Molding the Future of Ceramic Forming?xe2x80x9d, Oak Ridge National Laboratory Review, No. 4, 1995, pp. 25-39. Further information on gelcasting may be found in U.S. Pat. Nos. 4,894,194, 5,028,362, 5,145,908, 5,476,622, 5,401,445, and 5,419,860. Gelcast green parts have a relatively low binder content and are strong enough to be machined if necessary. Repeatable isotropic shrinkage allows gelcast parts to be made correspondingly larger than the desired fired parts, so that the parts shrink to the proper size during sintering. To make complex parts, gelcasting can be used either with fugitive or non-fugitive molds.
Non-fugitive-mold gelcasting typically uses metal molds, but since these are used at low pressures they will last longer than injection molds. However, metal molds are expensive, have significant leadtimes, and can only produce molded parts of limited shape complexity.
In fugitive-mold gelcasting, metal molds are used to produce wax molds which are then used for gelcasting. Since the metal tooling is only used with waxes it will last a long time, but the tooling is expensive and may have substantial leadtimes. The replication step from metal molds to wax molds also reduces the accuracy of the gelcast part.
By using rapid prototyping techniques to build molds for gelcasting one could eliminate the need for expensive, long-leadtime tooling. For information on making complicated gelcast structures using molds made by Fused Deposition Modeling (FDM), see the article by Jamalabad et al. xe2x80x9cGelcast Molding with Rapid Prototyped Fugitive Molds,xe2x80x9d Solid Freeform Fabrication Symposium Proceedings, Austin Tex., August 1996, pp. 71-78. However, conventional rapid prototyping techniques (including FDM) produce parts with a stairstep surface, and there is no way to smooth the internal surfaces of a mold made by such a process. Also because of material limitations, molds made by most rapid prototyping processes would be very difficult or impossible to remove from around gelcast green parts without damaging the green parts.
Many of the problems facing the above-described processes for making ceramic parts also affect the manufacture of complex polymer and metal parts. Current rapid prototyping methods of making complex polymer parts are limited in the types of polymers that can be used. Parts made by conventional rapid prototyping processes also suffer from stairsteps and other geometrical artifacts which reduce surface quality.
It is a primary object of the present invention to provide a method of making parts having complex shapes including internal passages and undercuts. It is another object to allow making complex parts with high accuracy, in particular without requiring multiple replication steps. It is another object to provide a method of making molded parts having improved surface finish. It is another object to allow the manufacture of complex monolithic parts without interlayer boundaries. It is another object to provide a method allowing the manufacture of complex molded parts of a relatively wide range of materials, including materials having superior mechanical properties. The present invention allows the use of mold/support material combinations not possible with other layered manufacturing processes, including solvent-soluble wax mold materials and water-soluble support materials for molds. The present invention also allows the manufacture of complex parts made of materials which do not adhere to themselves (i.e. sequentially deposited layers would not adhere to themselves). It is another object to provide a method allowing for the removal of molds from around molded parts without the application of force, thus allowing the production of undercut or fragile parts which would be otherwise either nonremovable or easily damaged. It is another object to provide a method for removal of molded parts from molds which requires no handwork. It is yet another object to provide a method of making heterogeneous molded parts containing multiple materials (provided the materials have compatible sintering or processing requirements), such as polymer/polymer, ceramic/ceramic, metal/metal, or ceramic/metal parts.
The above objects and advantages are provided by the following method of making molded parts. The present method begins with sequentially depositing and shaping a number of mold layers. At least one of the mold layers includes a support segment made of support material. This process of depositing and shaping the mold layers produces a mold filled with support material. The support material is removed by liquefying it (e.g., melting or dissolving the support material). This provides an empty mold cavity for receiving castable part material such as a gelcasting slurry. Casting part material into the mold produces a part having the shape of the mold cavity. The part is removed from the mold by liquefying the mold (e.g. melting or dissolving the mold).
The support material can be liquefied by melting or dissolving in a solvent which does not affect the mold material. The support material and mold material must be different so that the support material can be liquefied without affecting the mold material.
The shaping of the mold layers can be performed such that the mold has an undercut surface. This can be accomplished by depositing and shaping support material and then depositing overhanging mold material. Combinations of undercut and non-undercut mold segments or support segments provide a mold for complex parts.
The method of the present invention can provide molds for simultaneously producing a plurality of parts. In this embodiment, the support segments define a number of cavities in the mold corresponding to the different parts to be cast. Each cavity is filled with castable material. If desired, the different cavities are filled with different castable materials. Also, the different cavities can be interlocking. In this way, interlocking parts each made of different materials can be fabricated.